A well known use of storage phosphors is in the production of X-ray images. In U.S. Pat. No. 3,859,527 a method for producing X-ray images with a photostimulable phosphor, which are incorporated in a panel is disclosed. The panel is exposed to an incident pattern-wise modulated X-ray beam and as a result thereof the phosphor temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography”.
The image quality that is produced by any radiographic system using a phosphor screen, thus also by a digital radiographic system, largely depends on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be.
This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimum sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapor deposition, which may be thermal vapor deposition, sputtering, electron beam deposition or other of phosphor material on a substrate. However, this production method can not be used to produce high quality screens with every arbitrary phosphor available. The mentioned production method leads to the best results when a phosphor is used the crystals of which melt congruently.
The use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and congruent melting of these phosphors makes it possible to manufacture structured screens and binderless screens.
It has been disclosed that when binderless screens with an alkali halide phosphors are produced it is beneficial to have the phosphor crystal deposited as some kind of piles, needles, tiles, or other related forms. So in U.S. Pat. No. 4,769,549 it is disclosed that the image quality of a binderless phosphor screen can be improved when the phosphor layer has a block structure, shaped in fine pillars.
In U.S. Pat. No. 5,055,681 a storage phosphor screen comprising an alkali halide phosphor in a pile-like structure is disclosed. The image quality of such screens still needs to be increased and in JP-A-06/230 198 it is disclosed that the surface of the screen with pillar like phosphors is rough and that a levelling of that surface can increase the sharpness. In U.S. Pat. No. 5,874,744 the attention is drawn to the index of refraction of the phosphor used in order to produce the storage phosphor screen with a needle-like or pillar-like phosphor.
In EP-A-1 113 458 a binderless storage phosphor screen is disclosed that comprises an alkali metal storage phosphor characterized in that said screen shows an XRD-spectrum with a (100) diffraction line having an intensity I100 and a (110) diffraction line having an intensity I100, so that I100/I110≧1. Such a phosphor screen shows a better compromise between speed and sharpness.
Upon excitation with high energy radiation, excitons or electron/hole pairs are created in prompt emitting phosphors and scintillators. In the subsequent recombination of an electron and a hole, energy is released which is used for the creation of a luminescent photon, i.e. for the luminescence process. The presence of defects in the phosphor material gives rise to additional energy levels in the band gap. As a consequence, electrons can de-excite in many small steps. The resulting energy packets are too small to give rise to photon emission. Instead thereof the energy is transformed in so-called phonons or lattice vibrations. I.e. the excitation energy is lost in the form of heat.
In a similar way as in prompt emitting phosphors, high energy radiation creates electron/hole pairs in storage phosphors. In these materials, many electron/hole pairs do not recombine directly.
Instead thereof the electrons are trapped in electron traps and the holes are trapped in hole traps. Upon subsequent stimulation of the storage phosphor with light in the longer wavelength range as e.g. red light, the trapped electrons can absorb a photon. The photon supplies sufficient energy in order to escape from the trap. Such an escape is followed by recombination with a hole and by stimulated luminescence.
The traps in a storage phosphor are often intrinsic lattice defects. E.g. in alkaline earth halide and alkali halide storage phosphors, the electrons are trapped in halide vacancies, which are thus transformed into F-centres. If the storage phosphor crystal lattice is contaminated with foreign elements, additional defects are created. These defects can poison the luminescence as in a prompt emitting phosphor. In addition, these defects can compete with the intrinsic lattice defects as electron trapping centres. The additional defects are generally too unstable to be useful for long-term energy storage or too stable, so that the electrons are not released upon stimulation.
So, for prompt emitting phosphors and even more so for storage phosphors, it is of the utmost importance to avoid contamination with foreign elements.
Moreover high moisture content in the raw mix may cause troubles as bumping of the evaporation source which may occur as unacceptable inhomogeneities of the screens afterwards, while evaluating the quality thereof.
Many contaminations can be avoided by using very pure substances in the phosphor synthesis process. Other contaminations are more difficult to prevent.
Alkali halide and alkaline earth halide phosphors are often contaminated with oxides. The origin of this contaminating element may be water, adsorbed at the surface of the often slightly hygroscopic salt particles, more particularly at the surface of the Eu-compound derivatives. In the synthesis of the CsBr:Eu storage phosphor according to the state-of-the art methods the dopant material is the source of oxygen contamination.
In EP-A 1 276 117, synthesis of CsBr:Eu starting from CsBr and a Europium compound selected from the group consisting of Eu(II)halides, Eu(III) halides and Eu-oxyhalides is described as an improvement over using Eu2O3 as dopant material. It is clear that use of the above mentioned dopant compounds reduces the amount of the oxygen in the reaction mixture.
Yet, even use of europium halide EuXn (2≦n≦3) or europium oxyhalide (EuOX) may entail oxygen contamination. In the case wherein EuOX (X representing a halide) is used it is clear that oxide contamination will take place to a certain extent. As EuOX decomposes at a temperature of 700° C. or more (which represents a temperature, exceeding the melting temperature of CsBr:Eu with at least 100° C.) it is clear that the vaporisation process lacks for a “one phase” process from its initial step and that, when all of the starting materials are mixed in only one crucible, a phase separation occurs, further provoking instability in the vapor deposition process, the more as this phenomenon also causes bumping during said evaporation process and inhomogeneous deposit onto the phosphor support. A solution could be sought by strict separation of the raw stock materials in several (at least two) crucibles followed by vaporisation of raw materials or precursors from 2 crucibles or boats for the preparation of the dedicated phosphor, in such a manner that the resulting phosphor satisfies the stoichiometric requirements. Such a solution however requires strict geometrical arrangements within the vapor deposition chamber, and this may lay burden on the reproducibility of the process as the evaporation of the Cs-compounds and Eu-compounds proceeds after melting at differing temperatures.
Furtheron, even if a EuXn (2≦n≦3) material, without “structural” presence of oxygen at first sight, is used, however, oxygen contamination will take place unless very strict precautions are taken.
EuXn (2≦n≦3) compounds are known to be very hygroscopic. EuBr3 for instance is commercially available only as EuBr3.6-9H2O. When this material is heated, hydrolysis will take place and EuOBr is formed.
In order to avoid hydrolysis, dehydration must be complete, because presence of 1 molecule of water per molecule of EuBr3 is sufficient for complete transformation into EuOBr and HBr. Similar problems exist with other europium halides.
Hydrolysis and subsequent transformation into europium oxyhalide can be avoided if europium halide is heated to a temperature not higher than 200° C. under reduced pressure for a long time. For significant quantities, however, this process may take days or may even impossible to complete.
The resulting dehydrated europium halide will take up water, however, as soon as it is exposed to ambient atmosphere. This means that mixing with the CsBr matrix material must take place in a glove box or in a room with a conditioned, completely dry atmosphere. Also during transfer of the material to the reaction environment as e.g. a furnace to make powder CsBr:Eu or a vacuum chamber to make a CsBr:Eu phosphor layer by vapor deposition, precautions should be taken in order to avoid water take up.
Alternatively, the water containing raw mix, consisting of CsBr and dehydrated EuXn (2≦n≦3) can be dried in the reaction environment, i.e. in the furnace for production of CsBr:Eu powder or in the vacuum chamber for the production of CsBr:Eu layers by vacuum deposition.
However, drying a raw mix in a furnace is very time-consuming or even impossible, because the water must diffuse through a thick powder layer. Even for a limited thickness of the powder layer, the drying process may require several days, making the phosphor synthesis process very time consuming and inefficient.
When the raw mix is dried in the vacuum chamber in which vapor deposition should take place, a large amount of water vapor will be set free. This will disturb the vacuum and cause corrosion. Water will be readily adsorbed at the vacuum chamber walls and removal of the adsorbed water will again remain very time consuming.
In order to provide a method for manufacturing an europium halide molten and solidified body of high purity useful as a raw material for vapor deposition in particular, a method has been described in JP-A 2003-201119, wherein in the method for manufacturing the europium halide molten and solidified body, europium halide is molten by heating and then is cooled in the presence of a halogen source as e.g. ammonium halide, or a halogen as such, preferably under an atmosphere of dried air. In the presence of such compounds however corrosion may occur of environmental materials. Dryness processing during a heating time from 1 to 10 hours at temperatures up to 400° C. under vacuum moreover takes quite a lot of time.
Besides problems related with hygroscopy, corrosion, purity of the starting materials is not unambiguously provided as many undefined oxides may be present in differing ratio amounts and as moreover presence in crucibles of differing undefined “phases” may give rise to sputtering or bumping while vaporising the starting materials so that an unstable vapor flow and a non-uniform deposition may occur.